Intermontane Late Paleogene-Neogene Basins of the Andes of Ecuador and Peru: Sedimentologic and Tectonic Characteristics

An important characteristic of Neogene basin evolution in the Andean Cordillera was the formation of intermontane basins. These basins were initiated in the late Oligocene with reactivation of Andean tectonism and were abandoned in the latest Miocene (about 7 Ma). Their sedimentary fill and structures record the Neogene tectonic history. The sedimentary fill of these basins comprises two megasequences. The first consists of fluvial and overlying lacustrine deposits attributed to basin opening. The second is composed essentially of proximal fluvial sedimentary units and reflects the closure of the basins in the latest Miocene. Structural analysis of the Neogene basins shows that their evolution was controlled by the regional tectonic stress. Synsedimentary folding and fracturing show that the direction of stress experienced a clockwise rotation in the Neogene, thus explaining variations in the behavior of the faults bordering the basins as well as the different stages of their evolution.